Prematurity or immaturity?

Authors

  • PJ Steer

    Corresponding author
    1. Division of Surgery, Oncology, Reproductive Biology and Anaesthetics, Faculty of Medicine, Imperial College London, London, UK
      Prof PJ Steer, Department of Obstetrics and Gynaecology, Chelsea and Westminster Hospital, 369 Fulham Road, London, UK. Email p.steer@imperial.ac.uk
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Prof PJ Steer, Department of Obstetrics and Gynaecology, Chelsea and Westminster Hospital, 369 Fulham Road, London, UK. Email p.steer@imperial.ac.uk

Abstract

Birth in the human is particularly difficult compared with that in the other primates. Bipedalism has evolved over the past 6 or 7 million years, and has resulted in a small pelvis, adapted to the upright posture. In recent millennia, the increasing size of the fetal head at birth has made childbirth difficult. Haig has suggested that the mother and fetus do not have identical interests; the baby benefits from being large at birth, while it is easier for the mother to deliver a small baby. Many of the 500 000 maternal deaths per year around the world are due to obstructed labour, especially in Africa. Even in London, black African women have the highest caesarean section rates. Black African babies are on average smaller than white European babies, due mainly to earlier delivery. In a 13-year study of births in North West Thames, African babies were 2.5 times more likely to be born between 24 and 31 weeks inclusive than white European babies. Between 33 and 38 weeks of gestation, black African babies behaved in a more mature way, are more likely to pass meconium and have jaundice, but less likely to have respiratory problems leading to admission to the special care baby unit. Preterm black African babies have lower gestation-specific perinatal mortality than white European babies, while at term and beyond the reverse is true. Preterm birth may have evolved partly as a response to disproportion.

Introduction

Birth in the human is particularly difficult compared with that in the other primates. This difficulty has its origins in the fact that over the past 6 million years, a series of erect bipedal hominids evolved, of which we are the only current representatives. The changes in pelvic size and structure, which made bipedal locomotion possible were not a major problem until the past 150 000 years, when there has been a dramatic increase in the average size of the brain from about 500 cc in the Australopithecines, through 750 cc in Homo erectus, to greater than 1000 cc in modern humans.1–3 The increasing size of the fetal head at birth has made childbirth difficult.4,5 Haig has recorded that mother and fetus do not have identical interests; the baby benefits from being large at birth, while it is easier for the mother to deliver a small baby.6,7

A higher rate of obstructed labour in black Africans is associated with earlier delivery and accelerated fetal maturity

Many of the 500 000 maternal deaths per year around the world are due to obstructed labour, especially in Africa.8 Even in London, black African women have the highest caesarean section rates (Figure 1). This is associated with a narrower pelvis, efficient for running but less so for childbirth.9 Black African babies are on average smaller than white European babies, due mainly to earlier delivery (Figure 2). In a 13-year study of births in North West Thames, African babies were 2.5 times more likely to be born between 24 and 31 weeks inclusive than white European babies (PJ Steer, unpubl. obs.). Studies in Africa have confirmed that healthy black Africans have an average gestational length almost 1 week shorter than white Europeans.10 Between 33 and 38 weeks of gestation, black African babies behave in a more mature way than white European babies for the same gestational age. They are more likely to pass meconium11 (Figure 3) but are less likely to have jaundice or respiratory problems leading to admission to the special care baby unit (Figure 4).12,13 These differences in neonatal behaviour explain why black African preterm babies have a lower gestation-specific perinatal mortality than white European babies, while at term and beyond the reverse is true.14

Figure 1.

Caesarean section rates for different ethnic groups in London, 1998. Data from a pan London regional survey (81 091 births, 67 110 with ethnicity data).

Figure 2.

Gestational length at birth according to racial group (North West Thames database 1988–2000 inclusive, n = 503 003). Mean and 95% CI.

Figure 3.

Proportion of meconium staining of the amniotic fluid during labour, by racial group (white European n = 361 228; black African n = 16 888).

Figure 4.

Proportion of neonates transferred to the special care baby unit following delivery, by racial group (white European n = 362 073; black African n = 17 029).

The mechanisms leading to earlier delivery in black Africans are not established, but they are likely to include a higher rate of bacterial vaginosis,15 and a higher incidence of pre-eclampsia in the second trimester.16 This latter is of itself predictive of a better neonatal outcome at preterm gestations,17 as is also the case with pregnancy-induced hypertension.18

Preterm birth as a complex adaptation to disproportion

These findings suggest that preterm birth is a complex phenomenon, which may in part have evolved as a response to the conflict between walking and thinking, and a medical model of prevention is unlikely to be effective without a more detailed understanding of the varied and complex mechanisms of labour onset. Not all preterm babies are equally immature, and this needs to be taken into account when planning care. The best approach to prevention of prematurity is likely to be socio-economic (infection control, improved nutrition, avoiding smoking/alcohol/substance misuse, improving hygiene).